Probably the reason that happens is the three flanged wheels in each side have no "give" on that tight a curve and hold it where the pilot can't turn enough. I don't happen to have any O27 to put it on and see...
One remaining thing to check, though it requires removing the shell. The actual motor mount to motor screws are hidden inside the truck with the motor mounted. Those can also be loose and cause the same issue. I just fixed one with that problem.
Good point, you do have to take the truck motor mount screw out and then pull the motor out. The two screws that secure the motor mount to the motor are buried until the motor is out of the truck. I'm sort of a damn the torpedoes kind of guy.
If you have the shell off, you can usually just attempt to wiggle the motor, it should be solidly mounted on the truck, any movement between the motor and the truck is bad.